1. Field of the Invention
The present invention relates to a power polarity reversal protecting circuit, and more particularly to a protecting circuit which provides a protection to components in an integrated circuit so as to avoid damage caused by power polarity reversal.
2. Description of the Related Art
A requirement of requirement of protection from power polarity reversal connection is necessary in a driving circuit of a brushless cooling fan or a battery supplied electronic system. When a power polarity is reversed, a large current is passing through the integrated circuit and hence components in the integrated circuit are burned out. In order to avoid the aforementioned problems, many power polarity reversal protecting means are provided.
With reference to FIG. 3, an integrated circuit, such as an inverter, without any power polarity reversal protecting means is shown. The inverter includes a p-channel metal-oxide-semiconductor (PMOS) transistor (30), an n-channel metal-oxide-semiconductor (NMOS) transistor (31) and two parasitic diodes D1 and D2, wherein a connection on the PMOS transistor (30) and the NMOS transistor (31) is a known technology so it is not be discussed. The parasitic diode D1 has a cathode connected to a substrate of the PMOS transistor (30) and power VDD, and an anode is connected to an output of the inverter. The parasitic diode D2 has a cathode connected to the output of the inverter, and an anode is connected to a substrate of the NMOS transistor (30) and ground GND.
When the power polarity is correctly connected to the inverter, the inverter operates normally and the two parasitic diodes D1, D2 are both reversal biased between power VDD and ground GND. Once the power polarity is in reversal connection, it means that power VDD reverses to ground GND and ground GND reverses to power VDD, and the two parasitic diodes D1, D2 both become forward biased so that the large current is generated. The large current passes through the two parasitic diodes D1, D2 to make the components in the inverter ultimately burn out.
With reference to FIG. 4, in order to avoid damage caused by power polarity reversal, a protecting diode (40) is connected between the power VDD and the inverter. An anode of the protecting diode (40) is connected to the power VDD, and a cathode of the protecting diode (40) is connected to the inverter.
When the power polarity is correctly connected to the inverter, the protecting diode (40) is forward biased and the inverter operates normally. Once the power polarity is in reversal connection, the protecting diode (40) immediately becomes reversal biased and a conducting path through the two parasitic diodes D1, D2 is cut off so as to prevent the damage otherwise caused by power reversal connection. However, when the protecting diode (40) is forward biased, a constant voltage drop is generated across the protecting diode (40) and hence more power is dissipated. Furthermore, the protecting diode (40) is not set originally in the integrated circuit so an extra consumption of buying the protecting diode (40) is needed and hence the cost of the integrated circuit is raised.
To overcome the shortcomings, the present invention tends to provide a power polarity reversal protecting circuit for an integrated circuit to mitigate and obviate the aforementioned problems.
The main objects of the invention tends to provide a power polarity reversal protecting circuit for an integrated circuit so as to prevent damage caused by power polarity reversal connection.